1. Field of the Invention
The invention concerns a method and an apparatus for generation of a digital x-ray image of an examination subject.
2. Description of the Prior Art
In the acquisition of an x-ray image of an examination subject, for example in mammography, it must be ensured that the acquisition parameters (in particular the applied dose) are correctly set in order to ensure an image quality suitable for diagnostic evaluation. In order to limit the radiation exposure of the irradiated region of the examination subject to the diagnostically-required minimum, it is sought to already correctly set the acquisition parameters in the first image acquisition. For this purpose, an arrangement known as an exposure automatic (AEC, automatic exposure control) is used.
In the generation of an analog x-ray image with an x-ray film, for example in analog mammography, a number of solid-state detectors are arranged behind the x-ray film (in the direction of propagation of the x-rays). These detectors measure the intensity of the x-rays transmitted through the x-ray film, and the output signals of which are used to control the acquisition parameters (for example exposure time, operating voltage of the x-ray tube, x-ray current, anode filter combination).
Due to the higher absorption of the x-ray detectors used in digital x-ray imaging, such an exposure control ensuing during the image acquisition is, however, not possible. In x-ray apparatuses with digital x-ray detectors the exposure control ensues by making an exposure known as a pre-shot with a low dose in a first step. The dose radiated in the pre-shot is so low that detector and quantum noise influence the measurement signal respectively acquired by the individual detectors to a significant degree. The signals of a number of individual detectors are therefore combined (binning, undersampling) into respective measurement values, such that only a few hundred measurement values have to be evaluated instead of multiple tens of thousands (orders of magnitude more). These measurement values are used for determination of the correct exposure time or total dose. The data acquired in the pre-shot, however cannot be used for the diagnostic x-ray image (i.e., an image of sufficient quality to allow a competent diagnosis to be made there from) due to the low dose, and thus represent a small but nevertheless undesirable additional dose exposure.